1. Field of the Invention
This invention relates to a temperature sensor including a thermistor formed of a semiconductor such as a metal oxide or a metal resistor as a heat-sensitive device. More particularly, the invention relates to a temperature sensor that arranges a device inside a flow passage, through which a measured fluid (exhaust gas, for example) flows, inside a catalyst converter of an exhaust gas purification apparatus or inside an exhaust pipe of an automobile, and detects a temperature of the measured fluid.
2. Description of the Related Art
A known temperature sensor has a construction including a sheath member (sheath pin) with built-in metal core wires having a thermistor device as a temperature-sensitive device connected to a distal end side thereof and built-in lead wires for connecting an external circuit, connected on a rear end side thereof and a metal cap fitted to the sheath member while accommodating the thermistor device, wherein the sheath member is welded to a predetermined position of a flange (rib) (refer, for example, to Japanese Patent Laid-Open No. 162051/2000 (FIG. 1)). Such a temperature sensor is used as an exhaust gas temperature sensor for detecting a temperature of exhaust gas flowing inside an exhaust gas passage by the temperature-sensitive device.
3. Problems to be Solved by the Invention
The temperature sensor described in the above patent publication has a construction in which a sheath member is inserted into a flange and an end portion of the flange towards an exhaust gas passage (in other words, a distal end portion of the flange) is welded in an entire circumference by laser welding to fix the flange to the sheath member. In the temperature sensor having such a construction, however, the weld portion between the flange and the sheath member formed by laser welding is arranged inside the exhaust gas passage when an exhaust pipe is fitted, for example. Therefore, a problem of heat transfer to the flange from a heat-sensitive portion (portion on the side of a thermistor device arranged inside the exhaust pipe) through the weld portion develops inside the exhaust pipe. When heat transfer from the heat-sensitive portion to the flange becomes easier, the response and temperature-measuring accuracy of the heat-sensitive portion become deteriorated.
When a weld portion between metals is arranged in the exhaust gas passage, the weld portion is directly exposed to a high temperature environment and is oxidized. Consequently, durability of the sensor itself and air tightness of the exhaust gas are likely to be degraded in the course of use over a long term. To improve reliability of the temperature sensor, therefore, a construction is desired in which the number of weld portions of laser welding arranged inside the exhaust gas passage is as small as possible.
To solve the above problems of the prior art, on object of the present invention is to provide a temperature sensor having excellent durability, which is capable of suppressing heat transfer from a heat-sensitive portion to a flange, and having high reliability even when used under a high-temperature environment such as inside a catalyst converter of an automobile or inside an exhaust pipe.
The above object of the present invention has been achieved by providing a temperature sensor including a cylindrical metal tube having a distal end thereof closed and extending in an axial direction; a device accommodated in the metal tube and having electric characteristics thereof changing in accordance with temperature; and a flange arranged so as to encompass an outer peripheral surface of the metal tube; wherein the flange includes a sheath portion extending in the axial direction and a projection portion positioned on a distal end side of the sheath portion and protruding outward in a diametric direction, the metal tube is pushed into, or clamped and fixed to, at least the sheath portion, and the metal tube and the sheath portion are welded in a circumferential direction.
In the temperature sensor according to the invention, the metal tube and the flange are welded integrally with each other. This welding is made at the sheath portion positioned on the rear end side of the projection portion but not at a portion facing the inside of the flow passage through which the measured fluid flows, such as the exhaust gas passage inside the flange (more concretely, not on the distal end side of the projection portion). Therefore, when the temperature sensor is fitted to a flow passage tube through which the measured fluid flows, the weld portion for fixing the flange and the metal tube is not arranged inside the flow passage. In other words, the weld portion between the flange and the metal tube is disposed at a position at which the weld portion is not exposed to the measured fluid such as the exhaust gas. Consequently, a heat transfer path extending from the heat-sensitive portion to the flange through the weld portion is not formed. Furthermore, the degree of heat transfer from the heat-sensitive portion to the flange can be reduced much more than in the prior art sensors, and the effect of improving sensor response and preventing a reduction in temperature-measuring accuracy can be achieved. Furthermore, because the invention employs a construction in which the metal tube itself is welded and fixed to the sheath portion of the flange, the weld portion is not exposed to the measured fluid as described above, and reliability of air tightness to the measured fluid can be improved.
In the invention, the metal tube for accommodating the device and the sheath portion are welded in the circumferential direction while the metal tube is pushed into, or clamped and fixed to, at least the sheath portion of the flange. Therefore, welding strength is high, and adhesion strength between the flange and the metal tube is also high. The temperature sensor according to the invention has high durability even when used under a severe environment such as vibration of an automobile, and can further improve reliability of air tightness to the measured fluid.
When the temperature sensor is used for detecting the exhaust gas temperature of the automobile, it is used in a high-temperature environment of 200 to 1,000° C. Therefore, not only the outer surface of the metal tube but also its inner surface is oxidized and the oxygen concentration inside the space for accommodating the device remarkably drops. Consequently, the surface of the device is reduced with the result that the characteristics of the device change. This oxidation is likely to occur particularly on the outer and inner surfaces of the weld portion between the metal tube and the flange. When such a weld portion is formed on the distal end side of the flange facing the inside of the flow passage, the weld portion itself is exposed to the high-temperature environment and oxidation is further promoted. In the invention, on the other hand, the metal tube and the flange are welded not at the projection portion on the distal end side of the flange facing the inside of the flow passage but at the sheath portion positioned on the rear end side of the projection portion. Consequently, the occurrence of oxidation at the weld portion can be suppressed and the temperature sensor has high durability.
In the temperature sensor described above, the sheath portion constituting the flange has a two-step shape including a distal end step portion positioned on the distal end side and a rear end step portion having an outer diameter smaller than that of the distal end step portion. The metal tube may well be welded to the rear end step portion of the sheath portion.
To form the weld portion between the metal tube with the sheath portion of the flange with sufficient welding strength in the circumferential direction of the sheath portion, it may be conceivable to employ a method that sets the welding condition to a higher level, or to reduce the thickness of the sheath portion and to conduct welding without changing the welding condition. When the welding condition is merely increased, however, the cost will rise. When the thickness of the sheath portion is reduced as a whole, on the contrary, mechanical strength of the sheath portion itself will decrease. Therefore, the invention forms the sheath portion of the flange into a two-step shape having a distal end step portion and a rear end step portion having a smaller diameter than the distal end step portion, and welds the metal tube to the rear end step portion of the sheath portion. In other words, the thickness of the weld position of the sheath portion that is to be welded is small. In this way, the invention can satisfactorily weld the sheath portion and the metal tube, and can secure sufficient welding strength between them in addition to sufficient mechanical strength of the sheath portion and eventually, mechanical strength of the flange. Additionally, the diameter of the rear end side of the sheath portion is preferably smaller than that of the distal end side because machining becomes easier than when the rear end side has a greater diameter than the distal end side.
In any of the temperature sensors described above, welding between the metal tube and the sheath portion of the flange is not particularly limited. Exemplary welding technologies include laser welding, plasma welding, electron beam welding and argon welding.
Any of the temperature sensors described above preferably has a construction which further includes a sheath member having built-in metal core wires having the device connected to a distal end side thereof and built-in lead wires for connecting an external circuit, connected to a rear end side thereof, and a cylindrical joint bonded air-tight outside the sheath portion of the flange in the diametric direction and extending rearward in the axial direction, and wherein the distal end side of the sheath member is inserted into the metal tube, and the rear end side of the metal tube and the distal end side of the lead wires are arranged inside the joint.
In the temperature sensor according to the invention, the device accommodated in the metal tube and the lead wires for connecting the external circuit are connected to one another through the sheath member having the built-in metal core wires. Therefore, a step of separately packing insulating powder between the metal tube and the lead wires is not necessary and their electrical insulation can be reliably established. In the invention, while the distal end side of the sheath member is inserted into the metal tube, the rear end side of the metal tube is arranged inside the joint separately bonded to the rear end side of the flange, and the distal end side of the lead wires is arranged inside the joint. Therefore, the device is accommodated inside the closed space defined by the metal tube, the flange and the joint as the metal enclosure members while ventilation is secured by a ventilation path defined by the internal space of the lead wires, the internal space of the joint and the space between the outer peripheral surface on the distal end side of the sheath member and the inner peripheral surface of the metal tube.
Therefore, even when the inner surface of the metal tube is oxidized in the invention, a reduction in oxygen concentration inside the metal tube can be suppressed because ventilation is secured between the outside and the inside of the metal tube. Consequently, the change of characteristics of the device resulting from oxidation can be suppressed. Additionally, means for welding the joint and the flange are not particularly limited, and laser welding, plasma welding, electron beam welding, argon welding or brazing can be employed.
In the temperature sensor having the construction described above, the sheath portion has a two-step shape including a distal end step portion positioned on a distal end side and a rear end step portion having an outer diameter smaller than that of the distal end step portion, and the metal tube is welded to the rear end step portion of the sheath portion, and the joint is bonded to an outer peripheral surface of the distal end step portion in a circumferential direction.
As described above, when the sheath portion of the flange has a two-step shape including the distal end step portion and the rear end step portion having a smaller diameter than the distal end step portion and the metal tube is welded to the rear end step portion of the sheath portion, welding strength can be sufficiently secured between the sheath portion and the metal tube while mechanical strength of the flange can be secured. In the temperature sensor of the invention, the cylindrical joint is bonded to the outer peripheral surface of the distal end step portion of the flange. Therefore, the weld portion between the rear end step portion of the sheath portion of the flange and the metal tube is accommodated in the joint. Consequently, the joint plays the roles of preventing brine and moisture from adhering to the weld portion between the metal tube and the flange, and preventing the weld portion from being corroded by moisture or the like.
The present invention also provides a temperature sensor including a sheath member that includes built-in metal core wires having a device having electric characteristics thereof changing in accordance with temperature, the device being connected to a distal end side of the core wires, and built-in lead wires for connecting an external circuit, connected to a rear end side thereof; a cylindrical metal cap having a distal end thereof closed, extending in an axial direction and having an inner periphery on the rear end side thereof connected to an outer periphery on the distal end side of the sheath member in a circumferential direction while accommodating therein the device; and a flange so arranged as to encompass the outer periphery of the sheath member; wherein the flange includes a sheath portion extending in the axial direction and a projection portion positioned on the distal end side of the sheath portion and protruding outward in a diametric direction, the sheath member is pushed into, or clamped and fixed to, at least the sheath portion, and the sheath member and the sheath portion are welded in a circumferential direction.
In the temperature sensor according to the invention, the sheath member having the metal cap for accommodating the device bonded thereto is integrally welded to the flange. This welding is applied at the sheath portion positioned on the rear end side of the projection portion but not at a portion facing the inside of a flow passage through which a measured fluid flows, such as an exhaust gas passage inside the flange (more concretely, not on the distal end side of the projection portion). Therefore, when the temperature sensor is fitted to the flow passage through which the measured fluid flows, the weld portion for fixing the flange and the sheath member is not arranged inside the flow passage. In other words, the weld portion between the flange and the sheath member is disposed at the position at which it is not exposed to the measured fluid. As a result, a heat transfer path extending from the heat-sensitive portion to the flange through the weld portion is not formed inside the flow passage. Furthermore, the degree of heat transfer from the heat-sensitive portion to the flange can be reduced much more than in the prior art sensors, and the effect of improving sensor response and preventing a reduction in temperature measuring accuracy can be achieved. In comparison with temperature sensors having the prior art construction, the number of the weld portions between the metals that face inside the flow passage can be decreased. Consequently, the occurrence of oxidation at the weld portion can be suppressed and air tightness to the measured fluid can be improved.
In the invention, while the sheath member is pushed into, or clamped and fixed to, at least the sheath portion of the flange, the sheath member and the sheath portion are welded in the circumferential direction. Therefore, welding strength is high, and adhesion strength between the flange and the sheath member is also high. Even when used in an environment having vigorous vibration, the temperature sensor according to the invention exhibits high durability and can further improve reliability of air tightness to the measured fluid.
In the temperature sensor described above, the sheath portion constituting the flange has the two-step shape including the distal end step portion positioned on the distal end side and the rear end step portion having a smaller outer diameter than that of the distal end step portion, and the sheath member is preferably welded to the rear end step portion of the sheath portion.
To form the weld portion with sufficient welding strength between the sheath member and the sheath portion of the flange in the circumferential direction of the sheath portion, it may be possible to set the welding condition to a higher level, or to reduce the thickness of the sheath portion without changing the welding condition and to conduct welding. When the welding condition is merely increased, however, the cost will rise. When the thickness of the sheath portion is reduced as a whole, on the contrary, mechanical strength of the sheath portion itself is reduced. Therefore, the invention forms the sheath portion of the flange into the two-step shape having the distal end step portion and the rear end step portion having a smaller diameter than the distal end step portion, and welds the sheath member to the rear end step portion of the sheath portion. In this manner, the invention can satisfactorily weld the sheath portion and the sheath member, and can secure sufficient welding strength between them and also sufficient mechanical strength of the flange. Additionally, exemplary means for welding the sheath member to the sheath portion of the flange includes laser welding, plasma welding, electron beam welding or argon welding or the like.